I made finger paint last week at Playcentre using those ingredients.  Oh plus Lux flakes aswell.  From memory the recipe was
3C cornflour
1/2C lux flakes
1 1/2C cold water 
then add boiling water while rapidly mixing until it thickens into the consistency you are after. 
Add a bit of food colouring
I made pink, orange and green but by the end of session the kids had mixed it all together and the lux flakes make it all chunky so it looked like a big mess of brown chunky 

  lol!